Several compilers are "free" (but not "public-domain"). Most are under
GNU "copyleft", which means basically that they can be freely copied and
modified as long as modifications are distributed under the same scheme.

Contrary to a widespread myth, inexpensive Ada compilers are available.
They are especially inexpensive when you consider the amount of support
the language gives you with compile-time and run-time checking.

There is a growing list of validated Ada 95 compilers.
Note that the AJPO/AdaIC has closed (no more government funding).
Validations are still going on at two facilities, one of which (EDS)
publishes results.